The reason that this body-y hip-x/z rotation avoids gimbal lock is obvious.  With only one or two rotations, gimbal lock can never occur.  And since the BODY and hip are somewhat synonymous  (not the same thing, but usually the same center point for rotations), you have two places to do rotations.  Splitting the rotations 1-2 or 2-1 across the BODY and hip avoids the lock.

BODY is the top of each figure; think of it as the root body part.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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